Note: These examples will not magically create a "true" stereo output from the mono input, but simply place the same audio into both the left and right channels of the output (both channels will be identical).

2 × mono → stereo

Create a stereo output from two mono inputs with the amerge audio filter:

ffmpeg integrates a default down-mix (and up-mix) system that should be preferred (the -ac option) over the pan filter unless you have very specific needs.

If you want to map specific channels and drop the rest you can use the pan audio filter. This will map the FL (Front Left) of the input to the FL of the output, and the FR (Front Right) of the input to the FR of the output:

ffmpeg -i 6channels.wav -af "pan=stereo|c0=FL|c1=FR" stereo.wav

You can also map specific channels by number. This example will map the first and third channels of the input to the first and second channels of the output.

ffmpeg -i 6channels.wav -af "pan=stereo|c0=c0|c1=c2" output.wav

If the = in a channel specification is replaced by <, then the gains for that specification will be renormalized so that the total is 1, thus avoiding clipping noise. See the pan audio filter documentation for additional information and examples.

2 × stereo → stereo

Combine two stereo inputs into one stereo output with the amerge and pan audio filters:

Note: The output produced with the pan audio filter may not be identical to the output produced with -ac 2, so you'll have to listen to your outputs or view audio statistics to determine which output suits you.

A similar situation as above, but instead use the left and right channels from the first input to make the left channel out the output, and use the left and right channels of the second input to make the right channel of the output.